PROJECT SUMMARY Cancer, especially pediatric cancer, is a devastating diseases effecting a broad swath of humanity and consequently we must be able to understand the development of cancer in a holistic manner to develop the next generation of therapies. While there has been significant progress in elucidating genetic mutations which drive cancer initiation, many genetically engineered mouse models of cancer have indicated that necessary genetic mutations are oftentimes insufficient for tumor formation, i.e. not all cells harboring the necessary mutations form a tumor. Therefore, there must be other factors which license a cell with the capacity to form a tumor when the necessary mutations are present. We hypothesize that these cells are licensed as a consequence of the prior development of the cell. A cell's development is guided in large part by master regulators which are the key genes to define cell identity. As master regulators can induce cells to switch identity, a process known as reprogramming and which shares many characteristics to tumor initiation, we expect that misregulation of master regulators throughout development may be involved in the licensing of cells for cancer formation. We have shown that the master regulator, Sox2, is required for the formation of tumors that are initiated by the loss of the retinoblastoma tumor suppressor (Rb). Notably Sox2 is the master regulator to define the few cell types which form tumors when Rb is lost, most commonly neuroendocrine cells. We will therefore investigate the regulation of Sox2 during development in these neuroendocrine lineages and determine if misregulation of Sox2 is responsible for cancer licensing. We will pursue this investigation by pursuing the following aims: 1) To understand the mechanism of Sox2 regulation during development and how this goes awry in tumor formation, and 2) To determine if Sox2 derepressed cells are tumor initiating upon Rb-loss. We anticipate that the successful completion of these aims will lead to a greater understanding of how development might induce cancer later in life, and provide new avenues for cancer therapies and preventation.